1. Field of the Invention
The present invention relates to a liquid crystal composition, a selectively reflective film exhibiting selective reflection of light ranging from the ultraviolet region to the infrared region for use in a color filter and the like, and a method for producing the selectively reflective film.
2. Description of the Related Art
In recent years, attention has been drawn to liquid crystal materials, such as a cholesteric liquid crystal, that have a helical structure and exhibit selective reflection of a variety of colors depending on a twisting power (twist angle) of the helical structure. Since such liquid crystal materials are excellent in selective reflection and color purity of selectively-reflected light, they are widely used in various applications including optical films, liquid crystal color filters, recording media and the like.
Color filters (selectively reflective films) used in, for example, color liquid crystal displays are generally composed of red (R), green (G) and blue (B) pixels and a black matrix arranged therebetween for improving display contrast. Conventional color filters are mainly produced by dispersing pigments in a resin or by dyeing a resin with dyes and their production is ordinarily conducted by spin-coating a colored resin solution on a glass substrate to provide a colored resist layer followed by photo-lithographic patterning the resultant resist layer to form color filter pixels, or by directly printing colored pixels on a substrate.
The printing process described above has a problem in that color filters that are low in pixel resolution and hence unsuitable for forming detailed image patterns are produced. The spin-coating process has drawbacks, which cause large material loss and produce uneven coating when coating is applied to a large-area substrate. If a color filter is produced by an electro-deposition process, the obtained color filter has a relatively high resolution and reduced unevenness in colored layers, but there are drawbacks in that, for example, the production process is complicated and handling of processing liquids is difficult.
In light of the foregoing, there has been a demand for a method for effectively and readily producing a color filter having high quality with reduced material loss.
Color filters are required to have capabilities including high transmittance and high color purity. In recent years, attempts have been made to meet the above-mentioned requirements, for example, by suitably selecting types of dyes and resins to be dyed when dyes are used, or by using finely dispersed pigments when pigments are used. Recently, the level of performance required of color filters with respect to, for example, transmittance and color purity, are increasingly and extremely high when the filters are used in liquid crystal display (LCD) panels. Particularly, it is difficult for the color filters used in reflective LCDs to satisfy all of the requirements of good paper-white display, good contrast and good color reproducibility. Since color filters produced in a conventional manner, such as by dyeing a resin with dyes or dispersing pigments in a resin, are color filters of a light-absorbing type, color purity improvement obtained by increasing transmittance has almost reached its limit.
On the other hand, another type of color filter utilizing polarized light, which is mainly made of a cholesteric liquid crystal, is known. Since this type of color filter utilizing polarized light reflect light having predetermined wavelengths and transmit light of other wavelengths, light-utilizing efficiency is very high, and transmittance and color purity are highly remarkable as compared to color filters of the light-absorbing type. When such color filters utilizing polarized light are produced, spin-coating is typically employed to achieve evenness in layer thickness. However, large material loss is generated through spin-coating, making this production process disadvantageous in terms of cost.
In order to solve the above-described problems, a photoreactive chiral compound is effectively used for producing color filter films capable of exhibiting uniformity in color purity and requiring a reduced number of steps in a production process. When a liquid crystal composition containing a photoreactive chiral compound is patternwise irradiated with light having wavelengths to which the photoreactive chiral compound is photosensitive, the chiral compound causes a reaction, which progresses depending on the intensity of irradiated energy to induce a change in helical pitch (twist angle of the helix) of she liquid crystal compound. Through this process, desired selective color reflection can easily be obtained for each pixel merely by conducting patterning exposure using varied light quantities. This process for producing color filters is advantageous in that patterning exposure may be conducted only once using a photomask having a different light transmittance.
Thus, by conducting imagewise patterning exposure and subsequently fixing the cholesteric liquid crystal compound, a film capable of functioning as a color filter can be formed. This production process may be applied to an optical film, image recording, and the like.
When the liquid crystal composition is exposed to light of wavelengths to which the photoreactive chiral compound is photosensitive, selective reflection of light changes, for example, from B (blue) through G (green) to R (red), depending on the amount of the irradiated light. As shown in FIG. 2, when light of the color G (green) is reflected, a width (a2) of an amount of irradiated light is small, whereby the green light becomes bluish or yellowish if irregular exposure occurs at the time of irradiating light. Accordingly, it has been difficult to produce color filters exhibiting color uniformity.